A polarizing plate is typically produced by attaching a film mainly formed of cellulose triacetate as a protective film on both sides of a polarization film which is formed of iodine or a dichroic dye aligned and adsorbed to polyvinyl alcohol. Cellulose triacetate has features of being high in rigidity, frame resistance, and optical isotropy (low retardation value), and is widely used for the above-described polarizing plate protective film. A liquid-crystal display device is formed of a polarizing plate and a liquid-crystal cell. Today, TN-mode TFT liquid-crystal display devices, which are the main stream of the liquid-crystal display devices, realize high display visibility by inserting an optically-compensatory sheet (phase retardation film) between a polarizing plate and a liquid-crystal cell as described in JP-A 8-50206.
A cyclic polyolefin film is a center of attention as a film which is high in optical isotropy but can be improved in anisotropy, moisture absorbability, or moisture permeability by designing the molecular structure thereof, and is used for development of films to be used for polarizing plates and liquid-crystal display devices using heat fusion film formation or solution film formation. However, the heat fusion film formation has the problem that the optical characteristics in the width direction and the length direction (the retardation direction and the optical axis direction) change. It is generally known that the solution film formation provides a film having a better surface state. The optical anisotropy can be freely controlled during the solution film formation, by stretching the film in the width direction using a tenter, by stretching the film in the length direction between rolls, or by a combination thereof. The optical anisotropy is exhibited far more highly than when cellulose acylate film is used singly (in the state with no functional additives).
Recently, on the other hand, the liquid-crystal display devices are more strongly desired to have improved viewing angle characteristics. Optical transparent films, such as polarizer protective films, supports of optically-compensatory films, and the like, are desired to be more optically isotropic. For optical isotropy, it is important that a retardation value, which is represented by a product of the birefringence and the thickness of the optical film, be small. Especially in order to improve the display visibility when viewed in an oblique direction, it is necessary to reduce the thickness-direction retardation (Rth) as well as the in-plane retardation (Re). Specifically, it is necessary that, when the optical characteristics of the optical transparent film are evaluated, Re measured in the in-plane direction of the film is small and does not change even when measured at different angles.
Cellulose acylate films with a reduced in-plane Re are conventionally available, but it has been difficult to form a cellulose acylate film having a small Re change depending on the angle, i.e., having a small Rth.
As one solution, it is strongly desired to improve a cellulose acylate film which has an excellent level of adhesiveness to PVA, by further reducing the optical anisotropy thereof. Specifically, an optically isotropic, optical transparent cellulose acylate film which has an in-plane Re of almost zero and has a small retardation change depending on the angle, i.e., has an Rth of almost zero, is desired.
In production of a cellulose acylate film, a compound called a plasticizer is generally added in order to improve the film formation performance. As plasticizers, phosphoric acid triesters, such as triphenyl phosphate and biphenyldiphenyl phosphate, phthalates, and the like are disclosed (see, for example, Lecture of Plastic Materials, Vol. 17, “Cellulose Resins”, The Nikkan Kogyo Shimbun Ltd. (1970), page 121). Some of these plasticizers are known to have an effect of lowering the optical anisotropy of cellulose acylate films. For example, specific fatty acid esters are disclosed (for example, JP-A 2001-247717). However, these conventionally known compounds are not considered to have a sufficient effect of lowering the optical anisotropy of cellulose acylate films.
Recently, the liquid-crystal display devices are desired to have an improved level of displayed hue. In order to achieve this, the optical transparent films, such as protective films for polarizers, supports for optically-compensatory films, and the like, need to be reduced in Re and Rth in the visible light range having a wavelength of 400 to 800 nm, and also need to have small Re and Rth changes depending on the wavelength, i.e., a small wavelength-dependent Re and Rth distribution.
The liquid-crystal display devices are further desired to have no change in display performance, such as hue, contrast, viewing angle dependency, and the like, against a change in the environmental factors, such as humidity and the like, and thus provide high quality images.